Borehole telemetry system

ABSTRACT

A telemetry apparatus and method for communicating data from a down-hole location through a borehole to the surface is described including a light source, an optical fiber being placed along the length of the wellbore and receiving light from the light source, a transducer located such as to produce a force field (e.g. a magnetic field) across the optical fiber and its protective hull without mechanical penetration of the hull at the down-hole location, one or more sensors for measuring down-hole conditions and/or parameters, a controller to provide a modulated signal to the magnetic field generator, said modulated signal being under operating conditions representative of measurements by the one or more sensors, and an optical detector adapted to detect changes in the light intensity or polarization of light passing through the fiber.

The present invention generally relates to an apparatus and a method forcommunicating parameters relating to down-hole conditions to thesurface. More specifically, it pertains to such an apparatus and methodfor communication using an optical fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefits of priority from Application Number0524827.3, entitled “BOREHOLE TELEMETRY SYSTEM,” filed in the UnitedKingdom on Dec. 6, 2005, and which is commonly assigned to assignee ofthe present invention and hereby incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

One of the more difficult problems associated with any borehole is tocommunicate measured data between one or more locations down a boreholeand the surface, or between down-hole locations themselves. For example,communication is desired by the oil industry to retrieve, at thesurface, data generated down-hole during operations such as perforating,fracturing, and drill stem or well testing; and during productionoperations such as reservoir evaluation testing, pressure andtemperature monitoring. Communication is also desired to transmitintelligence from the surface to down-hole tools or instruments toeffect, control or modify operations or parameters.

Accurate and reliable down-hole communication is particularly importantwhen complex data comprising a set of measurements or instructions is tobe communicated, i.e., when more than a single measurement or a simpletrigger signal has to be communicated. For the transmission of complexdata it is often desirable to communicate encoded digital signals.

Widely considered for borehole communication is to use a direct wireconnection between the surface and the down-hole location(s).Communication then can be made via electrical signal through the wire.While much effort has been spent on “wireline” communication, itsinherent high telemetry rate is not always needed and very often doesnot justify its high cost.

Another borehole communication technique that has been explored is thetransmission of acoustic waves. Whereas in some cases the pipes andtubing within the well can be used to transmit acoustic waves,commercially available systems utilize the various liquids within aborehole as the transmission medium. Examples of the use of hydrauliclines for downhole power generation and telemetry are described in WO2004/085796 A1 and WO 2005/024177 A1.

Yet another borehole communication system is based on optical signals.Communication over an optical fiber is accomplished by using an opticaltransmitter to generate and transmit laser light pulses that arecommunicated through the optical fiber. Downhole components can becoupled to the optical fiber to enable communication between thedownhole components and surface equipment. Examples of such downholecomponents include sensors, gauges, or other measurement devices.

Typically, an optical fiber is deployed by inserting the optical fiberinto a control line, such as a steel control line, that is run along thelength of other tubing (e.g., production tubing). The control line isprovided as part of a production string that is extended into thewellbore.

As described for example in the published United Kingdom patentapplication GB 2409871 A, optical fibers can also be applied tointervention, remedial, or investigative tools as being deployed by awireline, slickline, coiled tubing, or some other type of conveyancestructure.

Further uses of optical fibers for communication inside a wellbore aredescribed in the related U.S. Pat. Nos. 5,898,517, 5,808,779 and5,675,674, which describe an optical fiber modulation and demodulationsystem using Bragg gratings and piezoelectric crystal combination.

However, a major limitation of conventional optical communicationssystems applied to hostile environments such as hydrocarbon productionwells is the need to terminate the fiber at each node of thecommunication system. The termination might be accomplished byconnecting the optical cable to the communication node, which involvesexpensive parts and lengthy procedures to ensure that the connection ishermetically sealed against the ingress of the downhole fluids.Alternatively, special optical connectors might be used that aresuitable for the hostile environment; however these are expensive. Inboth cases these connections, whether spliced or connectorised areexpensive and create a weak point that could degrade the overallreliability of the communications system.

Outside the technical field of borehole telemetry, Berwick M. and al.describe a magnetometer in their paper: “Alternating-current measurementand non-invasive data ring utilizing the Faraday effect in a closed-loopfiber magnetometer” Optics Letters Vol. 12. No. 4, 1987. Berwick M. andal. also propose to use the system as data ring. Similar methods andapparatus can be found in the U.S. Pat. Nos. 6,462,856 B1 and 4,996,692.

It is therefore an object of the present invention to provide opticalfiber based communication system that overcomes the limitations ofexisting devices to allow the communication of data into one or morenodes along the fiber without breaking into the fiber. The systemprovided is particularly for hostile environment where the fiber isenclosed in a protective tube or sheath. An example suitable for theinvention could be the communication between a down-hole location and asurface location.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, there is provided atelemetry apparatus and method for communicating digital data from adown-hole location through a wellbore to the surface. The apparatus ormethods includes a light source; an optical fiber being placed along thelength of the wellbore and receiving light from the light source,wherein the optical fiber is surrounded by a protective hull; one ormore transducers located to modulate optical properties of the opticalfiber interacting with the fiber so as to impart information onto thefiber without breaking into the protective hull at the downholelocation; one or more sensors for measuring down-hole conditions and/orparameters; a controller to provide a modulated signal to thetransducer, said modulated signal being under operating conditionsrepresentative of measurements by the one or more sensors; and anoptical detector adapted to detect changes in the properties of lightpassing through the fiber.

It is another aspect of the invention to provide apparatus and methodsfor modulating any one or any combination of these properties of thelight traveling through the fiber without penetrating the fiber orinterrupting its physical integrity of an protective hull, sheath ortube encapsulating the fiber at the point where the modulation isapplied. Hence no mechanical element of the transducer extends into orbeyond the boundary defined by the hull.

In a variant of the invention the fiber and the modulating transducerare separated without direct mechanical contact. In a preferredembodiment of this variant of the invention the modulating transducermodulates the light properties through a protective sheath or tube thatseals the tube from the environment without using or causing aperforation in the protective sheath or tube at the location ofmodulation. Thus, the fiber can be installed separately from thetransducer.

The transducer is preferably a magnetic field generator and even morepreferably a solenoid wound around the optical fiber or its protectivesheath or tube such that the fiber is preferably guided through the corearea of the solenoid.

The invention includes the variant of having several such transducersplaced along the length of the fiber thus creating a plurality ofcommunication nodes where data and information can be fed into thefiber.

The light transmitted through the fiber is preferably in a defined knownpolarization state, and more preferably linear polarized. In operationthe transducer may then changes a polarization state of the lightpassing through the fiber. In a variant of this embodiment, theinvention is making use of the Faraday effect.

In another variant of the invention, the transducer changes theamplitude, phase or frequency of the light preferably by causing amechanical force to act on the fiber. The section of fiber that isaffected by the transducer might also be modulated in its optical pathlength, the change being detectable preferably by interferometric means.

To enhance the effect of the transducer on the fiber, it is preferablyat least partially coated with hetero-material designed to respondspecifically to the force generated by the transducer. For example amagnetostrictive material may be used in the case of a magnetic fieldand a, preferably polymeric, piezo-electric coating in case of anelectrical field. Heat can also be used as a force field withtemperature induced changes of the optical properties of the fiber beingregistered at the surface.

In yet another variant, information is conveyed to the fiber by means ofacoustic waves that modulate the local refractive index of optical fibervia the stress-optical effect and thus modulate the optical path lengthof the fiber. Such changes in the optical path length can be convertedto measurable changes in the light, for example by interferometrictechniques.

Still another variant involves applying an electric field across thefiber and modulating its refractive index through the electro-opticeffect; the Kerr effect applies to all fibers and responds to the squareof the electric field; specially poled fibers are responsive linearly tothe electric field through the Pockels effect.

While the apparatus of the invention can be attached directly to casingor production tubing, it is regarded as a preferable placement method toguide the optical fiber through a control line attached to theproduction tubing with the transducer or transducers being placed suchthat the optical fiber inside the control line is within the forcefield.

The optical fiber may either form a loop from a wellhead to the downholelocation and returning back to the wellhead to guide light from thesource to the detector or may be terminated in the borehole with amirror.

It is further seen as advantageous to compensate for ambient drifts inthe detector signal through the use of a control loop preferably placedat surface. This control loop may include a modulator to change thepolarization of light passing through the fiber.

The invention further contemplates the use of a downhole power source toprovide a current for the magnetic field generator. If a battery orbattery pack is not suitable, the power source can be a generatorconverting for example pressure fluctuation, temperature gradients orvibrations of tubing into electrical power.

These and other aspects of the invention will be apparent from thefollowing detailed description of non-limitative examples and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates elements of an optical fiber telemetry system for awellbore in accordance with an example of the invention;

FIG. 2A shows details of an embodiment of the invention using a magnetfield ;

FIG. 2B shows details of a variant of the invention as shown in FIG. 2A;

FIG. 3 shows a signal generated using a method in accordance with anexample of the invention;

FIG. 4 schematically illustrates another embodiment of the invention;and

FIGS. 5A, B schematically illustrate another embodiment of the inventionusing a pressure field.

DETAILED DESCRIPTION

In a first example, the light propagating through an optical fiber isassumed to be polarized. The state of polarization at any locationinside the fiber refers to the variation of the electric field vector Eof the propagating light as a function of time. The most generalpolarization state is the elliptical polarization, but in the presentexample the light is assumed to be linear polarized. For a definition ofthe polarization state the electric field vector can be decomposed intothe superposition of two orthogonal fields. When the phase between thetwo vectors is 0 or π, the extremity of the electric field vectordescribes a line. The light is thus polarized linearly.

When light propagates through a given medium, the state of polarizationcan change and the material is then classified as birefringent. Forexample, in the case of a circularly birefringent material, the linearlypolarized light is strongly affected, whilst the circularly polarizedlight is unchanged in its state of polarization, although its velocityis dependent on whether the light is left- or right-hand circularlypolarized

The Faraday effect, which is known as such, is the induction of circularbirefringence in some materials by the application of a magnetic field.The circular birefringence induced in the fiber rotates the polarizationazimuth by an angle θ. The amount of rotation is expressed in terms ofthe Verdet coefficient V, which depends on the solid-state properties ofthe material, its temperature and the wavelength of the propagatinglight:θ=∫₀ ^(l) V{right arrow over (H)}·d{right arrow over (l)},   [1]where the integration is carried out over the length of fiber exposed tothe external magnetic field, H.

Therefore if the magnetic field is generated by a long solenoid carryinga current I wrapped N times around the fiber (ignoring ending effect),the expression of the angle of rotation can be approximated by:θ=VNI   [2]

This is the physical effect used for Faraday magnetometers. To detectthe variation of θ, in the polarization azimuth, a polarization analyzeris used.

It was found that the above-described Faraday effect can beadvantageously used for the purpose of this invention to transmitsignals from a location inside a wellbore to a surface location.

In FIG. 1 there is shown the schematics of a wellbore 10. The wellbore10 is lined with casing tubes 11. The lower part of the wellbore isshown with perforations 12 allowing the entry of produced fluids intothe wellbore. The top of the wellbore terminates in a wellhead 13.

Inside the wellbore 10 there is shown part of a production tube 14 toconvey produced fluids to the surface. The perforated section of thewellbore 10 is isolated from the remaining sections of the wellbore by apacker 15. Installed alongside the production tubing 14 is a (hydraulic)control line 16.

The control line is used to place an optical fiber 17 into the wellusing for examples fluid drag methods as disclosed in U.S. Pat. No. Re37,283, which patent is incorporated herein by reference. The fiber 17used in the example is a mono-mode or single-mode fiber known per se.

The example of FIG. 1 further shows a solenoid 18 surrounding thecontrol line 16, a module 19 including a power generator and acontroller to control the feeding current for the solenoid 18.

The power generator can be a suitable battery if communication isrequired only for a limited period of time. Otherwise the presentinvention contemplates the use of downhole power generators powered forexample through the hydraulic line 16. Details of such power generatorsare for example described in the above referenced international patentapplication WO 2005/024177 A1, incorporated herein by reference for allpurposes.

The module 19 is also connected to sensors 20 which are adapted tomeasure parameter or downhole conditions such as pressure, temperature,chemical composition, fluid properties, flow conditions and flowcomponents or the state of downhole components, such as control valves,packers and so on. On the surface there is shown further modules 21designed to project light into the fiber and control and measure thecharacteristics of the light which passed through the fiber. Details ofthe surface equipment 21 are shown in FIGS. 2A and 2B.

To the left side of FIG. 2A there is shown a light source, e.g. a laserdiode 22. The light emitted by the light source is polarized using apolarizer 221 and projected into the optical fiber 17 using a suitablemethod, which could be a lens 222 as shown.

Light thus fed into the fiber 17 forms a loop that at a downholelocation passed through the core of the solenoid 18 and returns to thesurface.

At the surface the light enters a beam-splitter 23 through lens 231. Thetwo beams of light emerging from the beam-splitter are each guidedthrough polarization filters 241, 242 and respective photodetectors 243,244. The output of the photodetectors 243, 244 is connected to afeedback unit 25 that computes the variation of θ as described above.The feedback unit provides also a controlled amount of current to thecompensation solenoid 26 that steers the polarization mode such that theoutput of the polarization filters 241, 242 is set in accordance withthe quadrature condition to be explained in further detail below.

In operation the analogue signal of the down-hole sensor 20 is digitizedinside the control module 19. An amplitude, frequency, or phasemodulated current corresponding to the obtained data sequence is thenapplied to the solenoid 18 through which the optical fiber passesaxially. This external variation in magnetic field varies thepolarization azimuth, θ of the propagating light via the Faraday effect.This change in θ is then detected at the surface via the polarizationanalyzer 21. The output signal is then demodulated via an amplitude orphase demodulation algorithm as appropriate.

In the polarization analyzer 21, the output light beam goes through thepolarizers 241, 242 oriented at ±45° with respect to the input lightbeam polarization axis, followed by the photo-detectors 243, 244. Thesignal power at each detector is therefore given by:P=P _(o)(1±cos 2(θ=θ₀)),   [3]where θ₀ is the offset angle between the original polarization axis andthe polarization azimuth of the output beam without any externalmagnetic field. The offset value θ₀ is due to the internal birefringenceof the fiber and the temperature gradient inside the wellbore. Thisoffset value and the Verdet coefficients are both temperature dependentand will drift. It is therefore difficult to measure absolute variationin θ. Alternatively the functions of 23, 241 and 242 can be combined ina polarizing beamsplitter, such as a Wollaston prism

However when following the above set-up the two photo-detector outputsare arranged in antiphase:i ₁ =P ₁(1+cos 2(θ+θ₀))i ₂ =P ₂(1−cos 2(θ+θ₀)),   [4]where θ₀, P₁, P₂ are constant. The signals i₁ and i₂ can be recombineddifferentially and by adjusting the gains a new output is obtained:i ₀≈cos 2(θ+θ₀).   [5]

This system response is most sensitive at:2(θ+θ₀)=π/4+2nπ  [6]This is the so-called quadrature condition.

In an ideal system, before the start of data transmission (but withlight propagating in the fiber 17), the polarization analyzers are setto satisfy the quadrature condition. However the drift in the offsetphase prevents the system from staying at the optimal quadraturecondition. Therefore an integration feedback loop using the second coil26 at the surface is used to restore the quadrature conditions. It willbe appreciated that the solenoid can be replaced by any other methodknown to change the polarization of the light beam such as Lefevreloops, mechanical manipulation (squeezing, twisting) and electro-opticalmodulation.

To overcome for example linear birefringence induced by bending in thefiber, the fiber may be twisted. Introducing a twist rate onto anoptical fiber is known to induce a fixed circular birefringence thatannihilates the unwanted linear birefringence effect. Further methods toimprove the output may include annealing the fiber.

The above example can be modified to include more fiber-based opticalcomponents to eliminate bulk optical components referred to.

In the example of FIG. 2B the laser source used is either a distributedfeedback or DFB semiconductor laser or a superluminescent light-emittingor SLD/SLED semiconductor laser diode 22. The DFB laser has very narrowoptical bandwidth (<1 MHz) and it is highly polarized optical sourcewith polarization maintaining fiber pigtail. The SLED source has verywide optical bandwidth (>35 nm) and it has single mode fiber pigtail.The output optical power is about 10 mW for both devices.

In order to eliminate any return signal, an optical isolator 222 with apolarization-maintaining fiber pigtail is introduced into the opticalcircuit. The SPFI-SS device offered by Micro-Optics Inc of Hackettstown,N.J., USA is, an example of a suitable device.

To increase the polarization extinction ratio from the optical source, afiber pigtailed polarizer 223 may be used. It has a single mode orpolarization-maintaining fiber at its input and polarization maintainingfiber at its output. For example, a fiber side-polished type ofpolarizer may be used and its polarization extinction ratio is about 23dB. Alternatively, devices based metal inserts in the fiber or coiledbirefringent fiber may be used. In certain instances, isolator 222 alsoincorporates a polarizer function. The plarizer 223 is set to generatelinear polarized at 45° from the principal axes of 224. In the case ofan all fiber system, this may be accomplished by splicing the outputfiber of the polarizer to the input of the coupler 224 such theprincipal axes of these two fibers are rotated at 45° from each other

A special polarization maintaining fiber coupler 224 (a suitable deviceis one from the PMC-IL-1×2 family provided by Micro-Optics Inc.) is usedhere. It is based on thin film technology and the polarizationextinction ratio is designed to be higher than 23 dB at both its fastand slow axes. The conventional fused-taper polarization maintainingfiber coupler could be used as an alternative with slightly lowerperformance (specifically, it cannot provide the same splitting rationon both polarization axes).

Behind the coupler 224 the light enters into the fiber 17 and passesthrough the core of the solenoid 18. The fiber is terminated at theremote end by a Faraday rotate mirror 225. The remote end of the fibercan be sited down the well, or brought up to the surface in a loopedcontrol line as described in the previous example.

The Faraday rotate mirror 225 is single mode fiber pigtailed and splicedto the normal single mode fiber 17. At room temperature it will makepolarization state change of 90° against its input. The actual statechange is however a function of temperature and operating wavelength.The mirror has a relatively narrow optical bandwidth (<20 nm) and alsoits operating temperature range is quite small (±5° C.). It may bereplaced by similar mirrors such as a fiber mirror or a fiber Bragggrating.

The polarization beam combiner 232 is also a fiber component based onthin film technology and it divides the x- and y-polarization componentsinto the separate output arms. A suitable device is, for example, one ofthe PDM-I1 family supplied by Micro-Optics Inc. The output of both armsis captured using sensitive photo-detectors such as 10 MHzadjustable-bandwidth balanced photo-receivers available as Model 2117supplied by New Focus Inc.

The 45°-angle splicing between two polarization-maintaining fiberscreates two orthogonal linear polarization components along its fast-and slow-axis. Both of them are launching into the PM coupler 224 andpropagate along the single mode down-lead fiber 17. The polarizationstate will change along the single mode fiber, however the returnedoptical signal will trace back along its original path with rotating90°-angle after it reflected from the Faraday rotate mirror. Thereforethe x- and y-polarization components swap the position after re-enteringthe PM coupler 224.

The result of a test of the system of FIG. 2B is shown in the FIG. 3,using a 2 km coiled fiber and a 1800 turn electro-magnetic coil and acommercially available polarization controller for adjustment of thepolarization state. The wire diameter is 0.56 mm, the length is 200 mand the resistance is measured as 16Ω. The average coil diameter isabout 35 mm and sensing fiber length is about 53 mm. Applying a 160 Hzmodulation frequency to the coil with a driving current of 0.45 A peakcurrent resulted in the shown single-shot measurement recorded with nofurther averaging. The gain of the balanced receivers has been set to3×10⁴ and the band-pass filter is set from 10 Hz to 1 kHz. In thisexperiment, the source power at the input to the isolator is 0.75 mW andthat reaching each input to the balanced receivers is 7 μW. In furthertests, it was found that readily detectable modulation on the opticalsignal was achieved with an electrical input to the coil below 35 mW.

It was found that the magnetic signals were transmitted through astainless steel control line without significant effect on themodulation depth.

The variations in a magnetic field or its gradient can also be sensedwith an optical fiber by using the induced dimensional change (i.e.strain) in a magneto-strictive element bonded to the fiber. This inducedstrain forces some light out of the fiber and thus results in a decreasein light intensity. This light intensity can then be modulated accordingto a recorded digital sequence to transmit data on the optical fiber. Atthe surface, the light intensity can be monitored by a photo detector.

In this example of the invention, as illustrated in FIG.4 an opticalfiber 41 is locally coated with a layer 411 of magneto-strictivematerial. In operation this part of the fiber 41 is located downhole inthe solenoid 42 similar to the apparatus described above. Permanentmagnets 421, 422 are located at each end of the solenoid 42. The magnetsare used to indicate an accurate placement of the coated part of thefiber 41 in the solenoid: A first change in the light intensity isregistered as the magneto-strictively coated fiber 41 passes the firstpermanent magnet 421. When the coated part of the fiber exits thesolenoid 42 and passes the second permanent magnet 422 a secondmodulation can be registered at the surface, thus indicating theaccurate placement.

In operation the current through the solenoid 42 will be controlled asdescribed above. However, in this embodiment changes in the magneticfield created by the solenoid are translated into a mechanical force onthe fiber and thus into a modulation of the light intensity, which ismonitored (and demodulated at the surface).

In a further variant of the invention, as shown in FIG. 5A the fiber 51-or a downhole section of the fiber, is formed into an interferometer,for example by providing a least two partial reflectors 511, 512 alongits length. Any modulation of the optical length between a reflectorpair may be read by a remote interferometer (not shown) which canconveniently be sited at surface. Fibers incorporating reflectors can beformed without significant changes in the external dimensions of thecoated fiber, for example, by inscribing gratings 511, 512 into thefiber 51. The spacing between reflectors 511, 512 may be selected toensure that just one, or several transducer modules 52 are locatedbetween the reflectors. The transducer 52 mounted on the outside of aprotective tube 53 which is turn is attached to a production tubing 54.The transducer 52 is a piezo-electric transducer using an acoustic horn521 generating acoustic waves 522 which travel through the protectivetube 53 and induces a pressure change inside which is largest in theregion between the gratings 511, 512. The acoustic wave generated by thesonic transducer 52 affixed to the control line 53 is focused by thehorn 521 inside the control line where the fiber resides. The pressureinduces a corresponding change of the optical path length L to L+ΔLbetween the second pair of gratings as schematically illustrated in FIG.5B. Optical fiber has a small, but detectable sensitivity to hydrostaticpressure and the sensitivity of the interferometric detection system issufficient for communications purposes.

The interrogation technique as illustrated in FIG. 5B is described ingreater detail but for other purposes by Dakin and Wade in PatentGB2126820 fully incorporated herein by reference.

If more than one pair of reflectors exists, then each can beinterrogated individually with minimal cross-talk. The inventors haveinterrogated arrays incorporating some 40 reflector pairs with betterthan 1:1000 cross-talk between any element in the array. Given thatfurther multiplexing of such arrays is possible using reflectorsoptimised for different optical wavelengths, it will be seen that thenumber of nodes of such a system is essentially unlimited.

Based on the above description, it will be appreciated by a skilledperson that any of the above effects which modulate the optical distancebetween the reflectors in a pair may be used either alone or incombination with other such methods to impart information onto thefiber.

Special coatings can be applied to the fiber to enhance the sensitivityof the fiber to an exposure to acoustic, magnetic or electric waves orfields such as the above-mentioned magneto-strictive coatings orpiezo-electric coatings in the case of electric fields. In the case ofelectric fields, it is also desirable to include in the control linewhich is generally metallic with a non-conductive section, which in turncan be placed in the electric field generated by a capacitor or dipole.The main direction the electrical field may be parallel or perpendicularto the axis of the optical fiber.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention, for example a temperaturegradient may be used as the force field described above. Changes of thetemperature modulate the optical properties across the protective hulland can be registered as signal on the surface.

1. A telemetry apparatus for communicating digital data from a down-holelocation through a wellbore to the surface, said apparatus comprising: alight source; an optical fiber being placed along the length of thewellbore and receiving light from the light source, wherein the opticalfiber is surrounded by a protective hull; one or more transducerslocated to modulate optical properties of the optical fiber interactingwith the fiber so as to impart information onto the fiber withoutbreaking into the protective hull at the downhole location; one or moresensors for measuring down-hole conditions and/or parameters; acontroller to provide a modulated signal to the transducer, saidmodulated signal being under operating conditions representative ofmeasurements by the one or more sensors; and an optical detector adaptedto detect changes in the properties of light passing through the fiber.2. The apparatus of claim 1 wherein the transducer includes a source ofa magnetic field, an electric field, a pressure wavefield, a temperaturefield or any combination thereof.
 3. The apparatus of claim 2 whereinthe magnetic field source is a solenoid is wound around the opticalfiber.
 4. The apparatus of claim 1 including one or more transducerslocated along the length of the fiber away from any terminals of thefiber.
 5. The apparatus of claim 1 wherein light entering the opticalfiber is polarized.
 6. The apparatus of claim 1 wherein the transducerin operation changes a polarization state of the light passing throughthe fiber.
 7. The apparatus of claim 1 making use of the Kerr effect orthe Faraday effect.
 8. The apparatus of claim 1 wherein transducerchanges the amplitude of the light.
 9. The apparatus of claim 1 whereinthe transducer causes a change in the optical path length through thefiber.
 10. The apparatus of claim 1 wherein the transducer generates afield causing a mechanical force to act on the fiber.
 11. The apparatusof claim 9 wherein the optical fiber is at least partially coated with amaterial specifically sensitive to the field.
 12. The apparatus of claim1 wherein the optical fiber is separated from the transducer by at leasta layer of fluid material.
 13. The apparatus of claim 1 wherein theoptical fiber is separated from the transducer by at least a layer offluid material and the protective tube surrounding the fiber at thelocation of the transducer.
 14. The apparatus of claim 13 wherein theoptical fiber is located within a control line and the transducer islocated at the exterior of the control line.
 15. The apparatus of claim1 wherein the optical fiber forms a loop from a wellhead to the downholelocation and returning back to the wellhead to guide light from thesource to the detector.
 16. The apparatus of claim 1 wherein the opticalfiber is terminated in the borehole with a mirror.
 17. The apparatus ofclaim 1 wherein the optical fiber is terminated in the borehole with aFaraday rotate mirror.
 18. The apparatus of claim 1 further comprising acontrol loop to compensate for ambient drifts in the detector signal.19. The apparatus of claim 18 wherein the control loop includes amodulator to change the polarization of light passing through the fiber.20. The apparatus of claim 18 wherein the control loop includes a beamsplitter to divide light passing through the fiber.
 21. The apparatus ofclaim 1 further comprising a power source in the wellbore.
 22. Theapparatus of claim 21 wherein the power source is a battery or agenerator.
 23. The apparatus of claim 21 wherein the power source is agenerator converting pressure fluctuation, temperature gradients orvibrations of tubing into electrical power.
 24. The apparatus of claim 1wherein the transducer is located outside the hull with no part, elementor connector penetrating the hull.
 25. A method of communicating digitaldata from a down-hole location through a wellbore to the surfacecomprising the steps of: letting light enter into an optical fiber beingplaced along the length of the wellbore inside a protective hull; usinga force field to modulate properties of the optical fiber at thedown-hole location without mechanical contact to the fiber and withoutbreaking into the protective hull at the downhole location; using one ormore sensors to measure down-hole conditions and/or parameters;providing a modulated signal to control the force field, said modulatedsignal being under operating conditions representative of measurementsby the one or more sensors; and detecting changes in the light intensityor polarization of light passing through the fiber.
 26. The method ofclaim 25 wherein the force field is a magnetic field generated using asolenoid.
 27. The method of claim 26 wherein the magnetic field isgenerated using a solenoid wound around the optical fiber.
 28. Themethod of claim 25 generating several force fields along the length ofthe fiber in the borehole.
 29. The method of claim 25 wherein lightentering the optical fiber is polarized.
 30. The method of claim 25wherein the force field in operation changes a polarization state of thelight passing through the fiber.
 31. The method of claim 25 using theKerr effect or the Faraday effect.
 32. The method of claim 25 comprisingthe step of modulating the optical path length through the fiber. 33.The method of claim 25 wherein the force field changes the amplitude ofthe light.
 34. The method of claim 25 wherein the force field causes amechanical force to act on the fiber
 35. The method of claim 25 whereinthe optical fiber is guided through a control line attached toproduction tubing.
 36. The method of claim 25 wherein the optical fiberforms a loop from a wellhead to the downhole location and returning backto the wellhead to guide light from a source to a detector.
 37. Themethod of claim 25 wherein the optical fiber is terminated in thewellbore with a mirror.
 38. The method of claim 25 using the furtherstep of compensating for ambient drifts in a detector signal.
 39. Themethod of claim 38 wherein the compensation step includes adjusting thepolarization of light passing through the fiber.
 40. The method of claim38 including the step of dividing light passing through the fiber intoat least two beams.